1. Field of the Invention
The present invention relates to an antenna device used to transmit and receive a radio signal, and particularly to an antenna device formed by simple combination of planar conductors including a radiating conductor and a ground conductor disposed to face each other with an insulating material interposed therebetween.
More specifically, the present invention relates to an antenna device of a planar structure mountable on a common printed board material or the like of a multilayer structure including layers of a conductor, a dielectric material, and a conductor, for example, and particularly to an antenna device of a planar structure which reduces the area of radiating conductors thereof and exhibits a wide band characteristic.
2. Description of the Related Art
In wireless communication using a radio wave communication method, a signal is transmitted with the use of a radiation field generated upon passage of current through an aerial (an antenna). The antenna has a variety of types. An antenna having a wide band characteristic can be used in communication which transmits and receives signals by diffusing the signals over an ultra wide frequency band such as a UWB (Ultra Wide Band). Further, a small-size antenna contributes to a reduction in size and weight of a wireless device.
In particular, an antenna configuration satisfying a request for a thinner antenna includes an antenna device configured such that a radiating conductor and a ground conductor plate are disposed to face each other with an insulating material interposed therebetween, i.e., a microstrip patch antenna (hereinafter abbreviated simply as the patch antenna). The shape of the radiating conductor is not particularly determined, but is rectangular or circular in most cases. The thickness of the insulating material interposed between the radiating conductor and the ground conductor plate is generally set to be equal to or less than one tenth of the wavelength of a radio frequency. Thus, the patch antenna can be formed into a substantially thin shape. Further, the patch antenna can be manufactured by an etching process performed on an insulating material substrate copper-clad on both sides thereof, and thus can be manufactured with relative ease. That is, it is relatively easy to manufacture the patch antenna.
For example, a magnetic microstrip patch antenna has been proposed in which short-circuiting conductor plates for making a radiating conductor and a ground conductor conductive are appropriately disposed at respective positions for suppressing excitation in an undesired mode, to thereby suppress disturbance in a radiation pattern at an end of a band, and in which a magnetic material having a relative permittivity of one or higher and having a multilayer structure including alternate lamination of a magnetic layer and an air layer is used to fill the gap between the radiating conductor plate and the ground conductor plate, to thereby realize unidirectivity in a wide bandwidth (see US Patent Application No. 2005/253756, for example).
A normal printed board has a structure in which a thin dielectric plate is vertically sandwiched by two conductor plates. If the printed board is structured such that the lower conductor plate is used as a ground (GND), and that the upper conductor plate is formed into a rectangular or circular shape and fed with electric power, a patch antenna can be formed and easily integrated with the circuit board.
FIGS. 10 and 11 illustrate a typical configuration example of the patch antenna formed on the printed board (FIG. 10 is a cross-sectional view of the patch antenna as viewed from a side, while FIG. 11 is a view of the patch antenna as viewed from obliquely above). In the printed board, the conductor layers 101 and 103 and 203 and 201 include copper or silver, for example, and the dielectric layer 102 and 202 includes a glass epoxy resin or Teflon (a registered trademark), for example. In the board structure as illustrated in FIGS. 10 and 11, in which the dielectric layer 102 and 202 is sandwiched by the conductor layers 101 and 103 and 203 and 201, a double-sided board is used. Alternatively, a multilayer board (e.g., alternate lamination of a conductor and a dielectric material) can also be used.
As illustrated in the drawings, the patch antenna can be viewed as an unbalanced feeding planar antenna, and is normally designed with an antenna formed by the upper conductor plate (a radiating element) regarded as a resonator. Further, current flowing along an end edge of the conductor plate is considered to be equal to current flowing through a parallel transmission line 104 extending across the dielectric material. Therefore, the patch antenna has a wavelength reduction effect according to the relative permittivity of the dielectric material. If it is assumed that a length L of the radiating element is equal to a width W of the radiating element, the patch antenna is designed on the basis of the following Equation (1).
                    L        =                  W          =                                    λ                              2                ⁢                                                      ɛ                    eff                                                                        =                                          λ                g                            2                                                          (        1        )            
Herein, ∈eff represents the effective permittivity of the dielectric substrate, and λg represents the effective wavelength. The effective permittivity ∈eff can be determined on the basis of the permittivity and a thickness h of the dielectric substrate and the value of the width W of the antenna (=the length L of the antenna). It is understood from the above Equation (1) that, if the length or width of the antenna (the radiating element) is reduced to half the effective wavelength λg, resonance occurs to radiate radio waves of a resonance frequency. Further, if a feeding point 204 is provided at a position offset from the center of the radiating element having the size W×L, the impedance matching can be achieved.
The effective permittivity ★eff of the dielectric substrate can be determined on the basis of the permittivity and the thickness h of the substrate and the value of the width W of the radiating element. Therefore, if the permittivity of the dielectric substrate is increased, the patch antenna can be reduced in size due to the wavelength reduction effect.
However, there is a limitation to the permittivity. Practically, it is necessary for the patch antenna to occupy an area of the size W×L on the printed board. This is because, in the patch antenna, the width W is increased to reduce the impedance of the antenna and thereby widen the band of the antenna. Therefore, the area of the antenna is increased.
Further, a planar patch antenna including a ground on the back surface thereof on a dielectric multilayer board generally has a narrow band (Current flowing along an end edge of a conductor plate forming a radiating element is considered to be equal to current flowing through a parallel transmission line extending across a dielectric layer. Further, the wavelength of the current is dominated by the relative permittivity of the dielectric material. That is, the frequency band of transmittable and receivable radio waves is limited to a narrow range dominated by a predetermined permittivity of the dielectric material). Frequency components which can be radiated by the patch antenna include a frequency f determined by the following Equation (2) on the basis of the effective wavelength λg described in the above Equation (1) and a higher harmonic component thereof. The frequency components do not represent a wide band.
                    f        =                  c                      λ            g                                              (        2        )            
In many of wireless communication techniques in the past, which assume long-distance communication, it suffices if only the behavior of the antenna in a far field is taken into account. In recent years, however, there have been increasing cases assuming close-range communication. Thus, it has been becoming necessary to understand phenomena occurring in a near field of the antenna, in which the communication distance is equal to or shorter than the wavelength.
It is now assumed that communication systems of recent years are divided into narrow band communication and wide band communication. The patch antenna generally tends to operate in a narrow band, and thus is considered to be unsuitable for, for example, a PAN (Personal Area Network) system, the operable band of which is necessary to be wide. Bandwidths having a VSWR (Voltage Standing Wave Ratio) of two or less are generally on the order of a few percent, depending on a design parameter. Due to this disadvantage, it is difficult to use the patch antenna in the wide band communication.
If the ground is provided on the back surface of the antenna on the dielectric multilayer board, the band of the antenna is narrowed. To ensure the wide band characteristic in the patch antenna of the related art, therefore, a structure not including the ground on the back surface of the antenna is generally employed. In such a case, however, the structure of a housing of an electronic device is complicated in design.